Multiple-stage flash evaporator

ABSTRACT

A multiple-stage flash evaporator in which the liquid to be distilled first passes through a heater located in the vapour space of a flash chamber in each of a plurality of stages of the evaporator and after being further heated subsequent to the last vapor space is introduced into the first flash chamber to flow through the flash chambers counter or to transversely of the flow of liquid being heated and partly flashes into a vapour formed in the flash chamber of one stage being passed uncondensed to another stage.

Aug- 15, 1972 R. s. slLvER MULTIPLESTAGE FLASH EVAPORATOR Filed Nev. 14,1969 INLET FOR INVENTOR 12035.21 Smps'ou Smvee. mlwm ATTORNE YS UnitedStates Patent O 3,684,662 MULTIPLE-STAGE FLASH EVAPORATOR Robert SimpsonSilver, Glasgow, Scotland, assignor to Weir Westgarth Limited, EastKilbride, Scotland Filed Nov. 14, 1969, Ser. No. 876,729 Claimspriority, application Great Britain, Nov. 16, 1968, 54,493/ 68 Int. Cl.B01d 1/28, 3/02, 3/00, /06, 3/10; C0211 1/04 U.S. Cl. 202-173 2 ClaimsABSTRACT OF THE DISCLOSURE A multiple-stage ilash evaporator in whichthe liquid to be distilled iirst passes through a heater located in thevapour space of a flash chamber in each of a plurality of stages of theevaporator and after being further heated subsequent to the last vaporspace is introduced into the iirst flash chamber to flow through theiiash chambers counter to or transversely of the flow of liquid beingheated and partly ashes into vapour in said chambers, a substantialproportion of the vapour formed in the flash chamber of one stage beingpassed uncondensed to another stage.

This invention relates to an improved multiple-stage ash evaporator ofthe kind in which feed liquid, such as brine, from which a distillate,such as fresh water, is to be distilled first passes through a heaterlocated in the condensing section located in the vapour space of a flashchamber in each of a plurality of stages of the evaporator, and afterbeing subsequently heated to the required temperature, is introducedinto the lirst flash chamber to flow through each flash chamber counterto or transversely of the dow of liquid being heated and partly flashesinto vapour in said chambers, the vapour part being condensed therein onsaid heater as a liquid distillate.

It was customary, heretofore, to cause substantially all the vapourproduced in the flash chamber of any stage to condense on a condensingsurface provided in that stage, although a small proportion, usuallyless than 1%, of the vapour was always uncondensed and passed to asubsequent stage.

It is an object of the present invention to render easier the removalfrom the evaporator of non-condensable waste gases.

The present invention is an evaporator, of the kind referred to,including means for passing a substantial proportion of the vapourformed in the flash chamber .of each stage uncondensed to another stage.

The proportion of vapour passed uncondensed from each stage is not lessthan and not greater than 25% of the vapour formed in that stage.

Preferably, means is provided for passing a proportion of the vapourformed in each stage to the next stage in sequence.

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying drawing which is adiagrammatic representation of a multiple-stage flash evaporator.

A flash evaporator is provided which is supplied with liquid, such asbrine, from which a distillate, such as fresh water is to be distilled.

The flash evaporator is of the kind which comprises a plurality ofstages 1, 2, 3 each of which has a flash chamber. The liquid firstpasses to be heated through a heater in the form of a tubular heatexchanger 4 located in the vapour space of each flash chamber. Afterbeing heated to a required temperature in a heat input section 5, theliquid is introduced into the flash chamber 6 of the rst stage 1. Partof the liquid in the flash chamber 6 of the iirst stage 1 flashes intovapour, say X lb./hr., the re- 3,684,662 Patented Aug. 15, 1972 icemaining non-distilled liquid being passed into the ash chamber 7 of thenext lower pressure stage 2.

Approximately of the vapour produced in the first flash chamber 1 isallowed to condense on a condensing surface 4 provided in the vapourspace of the first llash chamber 1, the liquid distillate produced beingled otf separately in any suitable manner (not illustrated) from theremaining non-distilled liquid. The remaining 25% of the vapour producedin the first flash chamber, say Y lb./hr. is fed without condensingthrough a device generally shown at 10 into the vapour space of the nextlower pressure ilash chamber 2, together with non-condensable gases.

In the next lower pressure flash chamber, the non-distilled liquidpassed from the previous flash chamber partly llashes into vapour, againsay X lb./hr.

Approximately 75% of the vapour produced is allowed to condense togetherwith the overliow vapour Y lb./hr. from the previous flash chamber onthe condensing surface 4 provided in the vapour space of the ash chamber7 of the second stage 2. The remaining 25 of the vapour again Y lb./hr.produced in the second dash chamber 7 is fed without condensing togetherwith non-condensable gases into the vapour space of the lower pressureflash chamber 8 and the non-distilled liquid remaining from the secondash chamber 7 is also passed separately into the next lower pressureilash chamber 8.

The construction and operation of the remaining stages is similar tothat of the second stage, approximately 25 of the vapour produced in astage being passed uncondensed together with the non-condensable wastegases to the vapour space of the next lower pressure stage, or, in thecase of the lowest pressure stage, drawn olf. The maintained velocity ofvapour and. non-condensable gases throughout the condensing regionallows the gases to be drawn olf more effectively.

Since a portion of the vapour in the `first stage is drawn olfuncondensed, here Y lb./hr., the temperature rise of the circulatingliquid in the tubular heat exchanger 4 of that stage is reduced ascompared with the temperature rise of the circulating liquid in each ofthe remaining stages. Assuming that the same quantity of liquid isflashed into vapour in each stage, the temperature rise of thecirculating liquid in each of the remaining stages is the same, that isgiven the same quantity of liquid flashed into vapour in each stage, andthe same quantity of circulating liquid passing through the tubular heatexchanger in each stage, the temperature rises in all the subsequentstages after the lirst are all substantially equal since each receivesthe percentage of vapour allowed to pass from the upper stage as well aslosing the percentage of vapour allowed to pass into the next lowerpressure stage. It is only the uppermost stage which has a netdeficiency because vapour is allowed to pass from it without anycorresponding amount entering from an upper stage. Hence the temperaturerise there is lower than in the others.

It is to be understood that the proportion of vapour produced in a stagewhich is passed uncondensed to the next stage or the actual quantity ofuncondensed vapour passed on may vary from stage to stage, but it isconsidered that the proportion which passes from one stage to the nextshould exceed 10% of the vapour formed in that stage, although incertain circumstances said proportion may only exceed 5% of the vapourformed in the stage. It is to be understood, however, that even wherethe proportion of uncondensed vapour which is passed from one stage toanother stage is as low as 5% of the vapour formed in said one stage,said proportion is, in the kind of evaporator with which the inventionis concerned, substantial as compared with the small proportion ofuncondensed vapour which is unavoidably passed from one stage to anotherin a `known evaporator.

Although the invention has been described with reference to anevaporator in which the circulating liquid is heated by passing itthrough all the heaters in the stages in sequence it may be used inconnection With other flash evaporators, such as disclosed in my priorBritish Patent No. 1,115,393.

I claim:

1. A multistage flash evaporator each stage of which comprises a flashchamber, a vapour space in the [flash chamber and a vapour condensingsection in said vapour space, a feed preheater and a distillatecollection means located in said condensing section supply means forliquid to be distilled connecting to the rst of said preheaters, a heatinput section in iuid communication with said preheaters, inlet meansfor supplying said heated liquid from said heat input section to the rst.'ash chamber, whereby the liquid partly llashes into vapour in eachchamber, and bleed means for passing not less than 5% and not greaterthan 25% of the vapour formed in the ash chamber of each stageuncondensed to the vapour space of the chamber of the next lowerpressure stage to increase the vapour and consequently thenoncondensable gas ow rate in said condensing spaces, and means forremoving the non-condensable gases and remaining associated vapor fromthe lowest pressure stage.

References Cited UNITED STATES PATENTS 3,099,607 `7/1963 Lustenader etal. 203-10 3,180,805 4/1965 Chirico 159-2 MS X 3,351,120 11/196'7Goeldner et al. l5913 B 3,206,379 9/1965 Hill 20'2-173 X 3,245,7624/1966 Ullrich et al 202-173 X iN-ORMAIN YUDKOFF, `Primary Examiner I.SOFER, Assistant Examiner UAS. Cl. X.=R.

